1. Field of the Invention
The present invention generally relates to the field of electronics, and more specifically, to thin film capacitors.
2. Description of the Related Art
Capacitors are a basic building block for electronic circuits. One design for capacitors is the parallel-plate configuration, in which a dielectric is sandwiched between two electrodes. FIG. 1 is a block diagram illustrating a typical metal-insulator-metal (MIM) parallel plate configuration of a thin film capacitor 100. The capacitor 100 is formed as a vertical stack composed of a metal base electrode 110b, a dielectric 120, and metal top electrode 110a supported by a substrate 130. The lateral dimensions, along with the dielectric constant and thickness of the dielectric 120, determine the capacitance value.
Materials in the barium strontium titanate (BST) family have characteristics that are well suited for use in such capacitors. BST generally has a high dielectric constant so that large capacitances can be realized in a relatively small area. Furthermore, BST has a permittivity that depends on the applied electric field. As a result, voltage-variable capacitors (varactors) can be produced, with the added flexibility that their capacitance can be tuned by changing a bias voltage across the capacitor. In addition, the bias voltage typically can be applied in either direction across a BST capacitor since the film permittivity is generally symmetric about zero bias. That is, BST typically does not exhibit a preferred direction for the electric field. One further advantage is that the electrical currents that flow through BST capacitors are relatively small compared to other types of semiconductor varactors.
Some devices utilizing BST capacitors require the application of a DC bias voltage for optimal performance. Examples of such devices include transmit/receive filters, impedance matching networks, delay lines, phase shifters, and antennas.
However, the voltage applied to BST materials can also induce a piezoelectric effect which can be detrimental to the device operation. Due to the intrinsic properties of the BST material, the electric field generated within the BST thin film 120 enables the conversion of electrical energy into mechanical energy. As the RF signal passes through the biased capacitor, part of its energy is converted into mechanical energy which then oscillates back-and-forth within the MIM structure forming a standing wave resonance. This standing wave will continue to draw energy away from the RF signal as long as the DC electric field remains. Consequently, this effect increases the transmission loss of the capacitor and leads to a degradation of overall device performance.
FIG. 2 is a graph illustrating RF transmission measurements, of the typical configuration of the capacitor 100, as a function of frequency of the RF signal voltage. Two curves are shown, corresponding to different applied DC voltage. At zero applied voltage, curve 210 shows a well-behaved response. By contrast, at an applied voltage of 20V, curve 220 shows a large resonance appearing at a specific frequency of 3.7 GHz caused by the piezoelectric effect.
As a result, there is a need for a system and method to decouple a frequency associated with a standing wave resonance of a capacitor from an operating frequency associated with an RF signal transmitted through the capacitor.